Fluorescent lamp inverter circuit

ABSTRACT

There is disclosed an inverter circuit particularly, but not exclusively, for operating fluorescent lamps. This circuit has a transformer to the primary of which is to be supplied a periodically switched D.C., a load being connected across the secondary. The transformer has an annular core having at least one air gap in it and two limbs projecting towards one another from opposing parts of the annulus and separated by an air gap. The primary and secondary windings surround the annulus on opposite sides of the limbs.

United States Patent Farrow [54] FLUORESCENT LAMP INVERTER CIRCUIT [72]Inventor: Victor Farrow, London, England [73] Assignee: ThornElectronics Limited, London,

England [22] Filed: June 19, 1968 [21] Appl. No.: 738,165

' [30] Foreign Application Priority Data Aug. 11, 1967 Great Britain.;.37,083/67 [52] US. Cl ..32l/45 R, 315/254, 336/165, 336/178 [51] Int.Cl. "H02!" 7/52 [58] Field of Search....315/98, 212, 219, 254,100 U,315/100 H, 100 T; 321/45; 323/892, 89.8;

[56] References Cited UNITED STATES PATENTS 3,008,068 11/1961 Wilting eta1.... ..315/100 'l [451 Nov. 21, 1972 1,353,711 9/1920 Bergman..336/l65 X 2,170,446 8/1939 Edwards ..336/165 X 2,949,565 8/1960Rohloff et a1 ..315/98 FOREIGN PATENTS OR APPLICATIONS 1,035,405 4/1953France ..3l5/l00 H 6,605,747 10/1966 Netherlands ..315/100 H PrimaryExaminerWilliam M. Shoop, .lr. Att0mey-Laurence Burns [57] ABSTRACTThere is disclosed an inverter circuit'particularly, but notexclusively, for operating fluorescent lamps. This circuit has atransformer to the primary of which is to be supplied a periodicallyswitched DC, a load being connected across the secondary. Thetransformer has an annular core having at least one air gap in it andtwo limbs projecting towards one another from opposing parts of theannulus and separated by an air gap The primary and secondary windingssurround the annulus on opposite sides of the limbs.

1 Claim, 7jDrawing Figures ACOUTPUT FLUORESCENT LAMP INVERTER CIRCUITThe present invention relates to inverted circuits, particularly, butnot exclusively, for use for operating fluorescent lamps. The invention,when applied to the operation of such lamps, is especially concernedwith arrangements in which the D.C. supply voltage is between 6 and 50volts, the commonest supply voltages used are 12 and 24 volts and themost popular wattage is about 40 watts. These parameters imply the useof fairly thick wire on the primary windings of the transformers (forinstance 18 or 20 s.w.g.). The frequency of operation-may be in therange of to 50 kc/s, but in most cases is likely to be about 20 kc/s asthis is suitably above audible frequency.

A typical inverter circuit of the kind discussed in the precedingparagraph is shown, in schematic form, in FIG. 1 of the accompanyingdrawings.

In FIG. 1, the terminals 10 and 11 are connected across a suitable D.C.supply. The circuit includes switching transistors V and V capacitors Cand C and resistors R and R connected as shown to the primary windingsof a transformer T The secondary winding of the transformer across whichan A.C. output is developed is connected to a load, in this example afluorescent lamp -12. I

The transformer T serves two purposes: the first is to step up theprimary voltage to a suitable level for operation of the lamp, and thesecond is to provide adequate leakage inductance to stabilize the lampcurrent. It must be appreciated that in most other circuits anadditional ballast component, either an inductor or a capacitor, isrequired to perform this stabilizing function.

An additional advantage of the circuit shown is that the cathode heatingvoltage V is high before the lamp strikes and falls when the lamp isalight. This is desirable because power is conserved when the lamp is inuse. To achieve the same result with other circuits involvesconsiderable complication.

FIG. 2 shows, somewhat schematically, a leakage reactance transformer,such as the transformer T in FIG. 1, in use at present. This comprises acore of square or rectangular cross section formed in two E- shapedparts, the center limbs carrying bobbins 1 and 2 respectively. The corematerial is usually manganesezinc or nickel-zinc ferrite and a gap 13 isprovided between the center limbs of the two parts. This gap determinesthe pre-strike frequency of the A.C. output. Bobbin 1 contains the wholeof the primary windings and bobbin 2 contains the whole of the secondarywindings. Adequate leakage reactance is obtained by virtue of the twobobbins being adjacent and coaxial.

In some cases wedges 14 of silicon iron are provided between the bobbinsbut this is of doubtful value since these iron wedges are very lossy athigh frequency and unless very carefully proportioned can substantiallyreduce the available pre-strike secondary voltage.

In the absence of a secondary load, the main magnetic flux set up by theprimary coil follows the path A. When the secondary coil is loaded,i.e., when the lamp strikes, this coil sets up an opposing m.m.f. Thetwo opposing magnetic fluxes meet in the center of the core and thebalance of the primary flux is compelled to return by the compositeferrite-air path B. In so doing it induces losses both in thesurrounding metal work and in the thick copper primary wires.

One practical effect of this is that it is not always possible to getpro-rata increases in efficiency by increasing the transformer size orwire gauges. For example, for 40 watt inverters in use at present thebest performance obtainable, compatible with general allroundperformance and economic mechanical design, is approximately 65 to 70percent. A large proportion of the losses are due to the transformerwhich will overheat unless special precautions are taken.

According to the present invention, there is provided an invertercircuit comprising a transformer and means for applying to the primarywinding of the transformer a periodically switched voltage derived froma D.C. supply when this is applied to input terminals of the circuit, anA.C. output being derived, in use, from the secondary winding of thetransformer, the transformer having an annular core provided with one ormore air gaps and with two limbs projecting toward one another fromopposing parts of the annulus and having an air gap between their ends,and primary and secondary windings surrounding the annulus on oppositesides of said limbs.

The invention will be described, by way of example, with reference toFIGS. 3 and 4 (ad) of the drawings in which FIG. 3 shows in elevation atransformer that may be used in a circuit as shown in FIG. 1 in carryingout the present invention, and

FIG. 4 shows at (a),.(b), (c) and (11) views in top plan, frontelevation, side elevation and underside plan respectively of one half ofthecore of the transformer of FIG. 3.

Referring to FIG. 3, this shows one embodiment of a transformer that canbe used in a circuit as shown in FIG. 1 in accordance with the presentinvention. The

core has a rectangular annular shape and the twobobbins 1 and 2 arearranged on the outer limbs, gaps Z/2 (making a total gap length of Z)are provided in the outer magnetic flux path linking the two bobbins,and gap of length y is provided between two center limbs C and D whichextend between opposing parts of the annulus and form a magneticshunt.,This magnetic shunt causes the opposing m.m.f. magnetic fluxes toby-pass most of the main winding. Some fringing will occur in the centergap y but this will only influence a small section of the coil windings.

The ratio of the gap y to the main circuit gap Z determines how muchsecondary pre-strike voltage is lost due to the effect of limbs C and D.However, owing to the physical make-up of the core a comparatively largey gap can be used whilst maintaining a good ratio of x/y. This is ameasure of the protection which the coils are getting from stray flux.

Arranging the coils on the outer limbs, as shown in FIG. 3, results in alarger cooling surface than with the arrangement of FIG. 2 and thewinding depth is smaller. The gaps of length Z/2 determine the prestrikefrequency. The gap length y may vary between 0.01 inch and x/2.

In FIG. 4 are shown views of one half of a core of the transformer ofFIG. 3 in a practical example. At (a) and (d) are top and underside planviews respectively, (b) is a view in front elevation and (c) is a viewin side elevation. It will be noted that the center limb C has beenextended in height (dimension h in FIG. 4(a), that is the dimension in adirection perpendicular to the while delivering 40 watts to the load andwhen powered by a 28-volt D.C. battery.

What is claimed is:

1. An inverter circuit comprising D. C. input terminals, A.C. outputterminals, a transformer, a coupling between said input terminals andthe primary winding of said transformer, switching means in saidcoupling, and connections between the secondary winding of saidtransformer and said output terminals, said transformer having anannular core, at least one air gap in said annular core, and two limbsprojecting toward one another from opposing parts ofsaid annular coreand having an air gap between their ends, said primary and secondarywindings surrounding said annular core on opposite sides of said limbs,and a primaryshunting condenser, wherein said limbs have a dimensiongreater than said annulus in a direction perpendicular to a planethrough the axes of said windings.

. k k II! III

1. An inverter circuit comprising D. C. input terminals, A.C. outputterminals, a transformer, a coupling between said input terminals andthe primary winding of said transformer, switching means in saidcoupling, and connections between the secondary winding of saidtransformer and said output terminals, said transformer having anannular core, at least one air gap in said annular core, and two limbsprojecting toward one another from opposing parts of said annular coreand having an air gap between their ends, said primary and secondarywindings surrounding said annular core on opposite sides of said limbs,and a primaryshunting condenser, wherein said limbs have a dimensiongreater than said annulus in a direction perpendicular to a planethrough the axes of said windings.